Devices to determine phase transitions are known, for example, from German Patent No. 1,648,964 and U.S. Pat. No. 4,557,152. Such devices are used to determine, in particular, the carbon content of steel manufactured in LD-converters. In general, the solidification chambers are manufactured from hollow or molded bodies made of a ceramic material, of resin-agglomerated sand, of metal, or of a combination of these substances. The basis of the invention is the probe known in the art from U.S. Pat. No. 4,557,152, wherein the solidification chamber is constructed as a hollow form made of fire-proof (refractory) molding sand, into which one or two metal plates are placed as cooling elements. The metal plates form part of the side walls of the chamber, while part of the side walls not covered with metal plates, as well as the front and rear walls of the chamber are bounded by the fire-proof molding sand.
Practical application of the probe has shown that, by using the known solidification chambers, a clearly recognizable critical point can only seldom be ascertained, which affects the accuracy of the carbon content determination. This is attributable, among other things, to the fact that a deviation of 1.degree.C. corresponds to a change of 0.013% in the carbon content. When taking measurements of steel with a carbon content from 0.02 to 0.2% C., this can lead to serious errors. In FIG. 1, the solid line represents a typical graph of the cooling curve of a steel sample obtained by using a solidification chamber made of a ceramic material, i.e., a material with poor heat-conductivity. Because of the poor heat conduction, the solidification temperature is only slowly reached, so that the critical (liquidus) point can only be ascertained with difficulty, and determining phase transitions in the solid state is practically impossible. In FIG. 1, the dotted line represents the curve obtained by using a solidification chamber made of metal, i.e., of a material with good heat-conductivity. Such graphs often illustrate a typical supercooling-phenomenon, which manifests itself by an S-shaped curve in the area of the solidification temperature and which makes the determination of the liquidus point substantially more difficult.